Inhibition of return is no hallmark of exogenous capture by unconscious cues

The Nature of Unconscious Attention to Subliminal Cues

Attentional selection in humans is mostly determined by what is important to them or by the saliency of the objects around them. How our visual and attentional system manage these various sources of attentional capture is one of the most intensely debated issues in cognitive psychology. Along with the traditional dichotomy of goal-driven and stimulus-driven theories, newer frameworks such as reward learning and selection history have been proposed as well to understand how a stimulus captures attention. However, surprisingly little is known about the different forms of attentional control by information that is not consciously accessible to us. In this article, we will review several studies that have examined attentional capture by subliminal cues. We will specifically focus on spatial cuing studies that have shown through response times and eye movements that subliminal cues can affect attentional selection. A majority of these studies have argued that attentional capture by subliminal cues is entirely automatic and stimulus-driven. We will evaluate their claims of automaticity and contrast them with a few other studies that have suggested that orienting to unconscious cues proceeds in a manner that is contingent with the top-down goals of the individual. Resolving this debate has consequences for understanding the depths and the limits of unconscious processing. It has implications for general theories of attentional selection as well. In this review, we aim to provide the current status of research in this domain and point out open questions and future directions.

The impact of temporal contingencies between cue and target onset on spatial attentional capture by subliminal onset cues

Prior research suggested that attentional capture by subliminal abrupt onset cues is stimulus driven. In these studies, reacting was faster when a searched-for target appeared at the location of a preceding abrupt onset cue compared to when the same target appeared at a location away from the cue (cueing effect), although the earlier onset of the cue was subliminal, because it appeared as one out of three horizontally aligned placeholders with a lead time that was too short to be noticed by the participants. Because the cueing effects seemed to be independent of top-down search settings for target features, the effect was attributed to stimulus-driven attentional capture. However, prior studies did not investigate if participants experienced the cues as useful temporal warning signals and, therefore, attended to the cues in a top-down way. Here, we tested to which extent search settings based on temporal contingencies between cue and target onset could be responsible for spatial cueing effects. Cueing effects were replicated, and we showed that removing temporal contingencies between cue and target onset did not diminish the cueing effects (Experiments 1 and 2). Neither presenting the cues in the majority of trials after target onset (Experiment 1) nor presenting cue and target unrelated to one another (Experiment 2) led to a significant reduction of the spatial cueing effects. Results thus support the hypothesis that the subliminal cues captured attention in a stimulus-driven way.

Dissociating the capture of attention from saccade activation by subliminal abrupt onsets

Attentional capture and effects on saccade metrics by subliminal abrupt onset cues have been studied with peripheral cues at one out of several (two to four) display locations, swiftly followed by additional onsets at the other display locations. The lead time of the cue was too short to be seen. Here, we were interested in whether such subliminal onset cues influenced saccades primarily by way of attention or by way of direct saccade activation. In separate blocks, participants made speeded pro-saccades towards a black target or anti-saccades away from the target. Prior to the targets, an abrupt onset cue was presented either at the same side as the target (valid condition) or at the opposite side (invalid condition). If cues influenced performance by way of attentional capture, we expected facilitation of target processing in valid compared to invalid conditions (cueing effect) in the pro- as well as in the anti-saccade task. If the cues activated saccades in their direction, we expected the cueing effect to drop in the anti-saccade task compared to the pro-saccade task because in the anti-saccade task the invalid cue would activate the finally required response, whereas the valid cue would activate the alternative response, leading to interference. Results were in line with the former of these possibilities suggesting that subliminal abrupt onsets influenced saccades by way of attention with no or little direct activation of saccades.

Unconscious processing of facial attractiveness: invisible attractive faces orient visual attention

Past research has proven human’s extraordinary ability to extract information from a face in the blink of an eye, including its emotion, gaze direction, and attractiveness. However, it remains elusive whether facial attractiveness can be processed and influences our behaviors in the complete absence of conscious awareness. Here we demonstrate unconscious processing of facial attractiveness with three distinct approaches. In Experiment 1, the time taken for faces to break interocular suppression was measured. The results showed that attractive faces enjoyed the privilege of breaking suppression and reaching consciousness earlier. In Experiment 2, we further showed that attractive faces had lower visibility thresholds, again suggesting that facial attractiveness could be processed more easily to reach consciousness. Crucially, in Experiment 3, a significant decrease of accuracy on an orientation discrimination task subsequent to an invisible attractive face showed that attractive faces, albeit suppressed and invisible, still exerted an effect by orienting attention. Taken together, for the first time, we show that facial attractiveness can be processed in the complete absence of consciousness, and an unconscious attractive face is still capable of directing our attention.

Unconscious vision and executive control: how unconscious processing and conscious action control interact

Research on unconscious or unaware vision has demonstrated that unconscious processing can be flexibly adapted to the current goals of human agents. The present review focuses on one area of research, masked visual priming. This method uses visual stimuli presented in a temporal sequence to lower the visibility of one of these stimuli. In this way, a stimulus can be masked and even rendered invisible. Despite its invisibility, a masked stimulus if used as a prime can influence a variety of executive functions, such as response activation, semantic processing, or attention shifting. There are also limitations on the processing of masked primes. While masked priming research demonstrates the top-down dependent usage of unconscious vision during task-set execution it also highlights that the set-up of a new task-set depends on conscious vision as its input. This basic distinction captures a major qualitative difference between conscious and unconscious vision.

Responding to the direction of the eyes: in search of the masked gaze-cueing effect

Recent studies have demonstrated that masked gaze cues can produce a cueing effect. Those studies, however, all utilized a localization task and, hence, are ambiguous with respect to whether the previously observed masked gaze-cueing effect reflects the orienting of attention or the preparation of a motor response. The aim of the present study was to investigate this issue by determining whether masked gaze cues can modulate responses in detection and discrimination tasks, both of which isolate spatial attention from response priming. First, we found a gaze-cueing effect for unmasked cues in detection, discrimination, and localization tasks, which suggests that the gaze-cueing effect for visible cues is not task dependent. Second, and in contrast, we found a gaze-cueing effect for masked cues in a localization task, but not in detection or discrimination tasks, which suggests that the gaze-cueing effect for masked cues is task dependent. Therefore, the present study shows that the masked gaze-cueing effect is attributed to response priming, as opposed to the orienting of spatial attention.

Oculomotor capture by supraliminal and subliminal onset singletons: the role of contrast polarity

According to a top-down explanation of subliminal oculomotor capture, only subliminal distractors with a contrast polarity matching that of the searched-for targets should capture attention. For instance, when looking for white targets only subliminal white but not black distractors should capture attention. In contrast, according to a bottom-up explanation of such capture effects, subliminal distractors with a contrast polarity different to that of the searched-for targets should also capture attention. For instance, even when looking for white targets, subliminal black distractors should capture attention. Here, we used subliminal singleton-onset distractors in the same vertical hemifield as the target versus singleton-onset distractors in the opposite vertical field to the target, and tested whether oculomotor capture by these distractors depended on a match between the searched-for target contrasts and the distractor contrasts, by measuring saccade latency, saccade trajectory deviation, and saccade endpoint deviation. We found evidence for oculomotor capture: subliminal distractors in the opposite field delayed saccade execution towards the target. This delay was found in comparison to subliminal distractors in the same hemifield as the target. In line with a bottom-up explanation, this delay was independent of the similarity between the distractor contrast polarity and the searched-for target contrast polarity. Together with the subliminality of the distractors, the experiment confirmed bottom-up oculomotor capture by subliminal singleton-onsets.

Enhancement and suppression in the visual field under perceptual load

The perceptual load theory of attention proposes that the degree to which visual distractors are processed is a function of the attentional demands of a task-greater demands increase filtering of irrelevant distractors. The spatial configuration of such filtering is unknown. Here, we used steady-state visual evoked potentials (SSVEPs) in conjunction with time-domain event-related potentials (ERPs) to investigate the distribution of load-induced distractor suppression and task-relevant enhancement in the visual field. Electroencephalogram (EEG) was recorded while subjects performed a foveal go/no-go task that varied in perceptual load. Load-dependent distractor suppression was assessed by presenting a contrast reversing ring at one of three eccentricities (2, 6, or 11°) during performance of the go/no-go task. Rings contrast reversed at 8.3 Hz, allowing load-dependent changes in distractor processing to be tracked in the frequency-domain. ERPs were calculated to the onset of stimuli in the load task to examine load-dependent modulation of task-relevant processing. Results showed that the amplitude of the distractor SSVEP (8.3 Hz) was attenuated under high perceptual load (relative to low load) at the most proximal (2°) eccentricity but not at more eccentric locations (6 or 11°). Task-relevant ERPs revealed a significant increase in N1 amplitude under high load. These results are consistent with a center-surround configuration of load-induced enhancement and suppression in the visual field.

Unconscious Cueing via the Superior Colliculi: Evidence from Searching for Onset and Color Targets

According to the bottom-up theory of attention, unconscious abrupt onsets are highly salient and capture attention via the Superior Colliculi (SC). Crucially, abrupt onsets increase the perceived contrast. In line with the SC hypothesis, unconscious abrupt-onset cues capture attention regardless of the cue color when participants search for abrupt-onset targets (Experiment 1). Also, stronger cueing effects occur for higher than lower contrast cues (Experiment 2) and for temporally, rather than nasally, presented stimuli (Experiment 3). However, in line with the known color-insensitivity of the SC, the SC pathway is shunted and unconscious abrupt-onset cues no longer capture attention when the participants have to search for color-defined targets (Experiment 4) or color-singleton targets (Experiment 5). When using color change cues instead of abrupt-onset cues, the cueing effect also vanishes (Experiment 6). Together the results support the assumption that unconscious cues can capture attention in different ways, depending on the exact task of the participants, but that one way is attentional capture via the SC. The present findings also offer a reconciliation of conflicting results in the domain of unconscious attention.